The present invention relates to a cruise control device capable of automatically changing the speed of a motor vehicle and maintaining it at a desired speed.
Among motor vehicles, those provided with a cruise control device are known. The cruise control device has the function of maintaining the speed of a motor vehicle, i.e., the vehicle speed, at a setting speed to provide automatic cruising, whereby the burden of operation on a driver can be reduced when the driver intends to run the vehicle at a constant speed. Namely, while the cruise control device is operating, the driver can enjoy cruising with his foot kept away from the accelerator pedal.
There have been proposed various manners of controlling the vehicle speed by the cruise control device. A generally known example is as follows. When vehicle speed V is to be increased or decreased toward target speed VO set by the driver, a control mode suited to one of control regions determined by the magnitudes of the difference between the target speed VO, and the current vehicle speed V and the current acceleration is applied so that the vehicle speed V is brought to the target speed VO and maintained. More specifically, as the control mode in the individual control regions, a so-called PID control (proportional plus integral plus derivative control) is employed, in which gains adapted to a P (proportional) term, I (integral) term, etc. are set for the individual control regions.
Although the gains of the PID control are individually set to different values in different control regions, the setting values are invariable. If the gains are fixed at constant values, it is not possible to carry out a cruise control matched to both the high- and low-speed regions. Therefore, when the vehicle speed V is adjusted to the target speed VO by operating the cruise control device, the driving feeling is sometimes damaged.
Further, there is a prior art cruise control device which employs a control mode wherein the acceleration of the vehicle is once set at 0 when the vehicle speed V is brought close to the target speed VO. According to this control mode, however, the vehicle speed V is temporarily held at a constant value before the vehicle speed V reaches the target speed VO. Therefore, not only the time required for the vehicle speed V to reach the target speed VO is prolonged, but the driving feeling before the target speed VO is reached is damaged.
The control action carried out by the above-described cruise control device is determinately started by a turning-on of a set switch (which is usually arranged near the driver's seat) by a driver at the time the vehicle speed V reaches the target speed VO. However, the manual handling of the vehicle when the control action is started differs depending upon drivers. Namely, some drivers may continue to depress the accelerator pedal for a while even after the turning-on of the set switch, whereas others may release the accelerator pedal before turning on the set switch. Moreover, a time lag from the time the control action is started by the cruise control device until the engine output changes in response to the control action, i.e., response delay, is inevitable.
If the variation in manual handling of the vehicle when the set switch is turned on, described above, is combined with the response delay of the engine, the control action of the cruise control device cannot be smoothly carried out, more specifically, frequent and unnecessary opening/closing of the throttle valve of the vehicle may be executed by the control action, thus damaging the driving feeling. This disadvantage is particularly prominent when the cruising speed of the vehicle is low or the vehicle is running down a slope.
To reliably ensure safety, the control action executed by the above cruise control device is interrupted simultaneously with an operation of the transmission or the brake pedal of the vehicle, otherwise, the acceleration or deceleration of the vehicle would be disabled even if the driver depresses the accelerator pedal after operating the shift lever of the transmission or depresses the brake pedal, thus making it impossible to avoid an urgent danger during cruising.
When an urgent danger is avoided after the control action of the cruise control device is terminated, it is desirable to resume the previous cruising condition, and to this end, a cruise control device has been proposed which is capable of controlling the operation of the transmission, automatic transmission of the vehicle in this proposed case.
Specifically, the cruise control device of this type is provided with a resume switch independent of the above-mentioned set switch, and when the resume switch is turned on, a shift command signal is output to the automatic transmission to resume the previous cruising condition. For example, when the resume switch is turned on after the brake pedal is depressed to temporarily decrease the vehicle speed, the cruise control device outputs a shift command signal to the automatic transmission to shift down the transmission, and upon passage of a predetermined time after the vehicle speed V reaches the target speed VO, outputs a shift command signal to shift up the automatic transmission, to thereby quickly increase the vehicle speed V toward the target speed VO.
As will be understood from the above description, the shift control of the automatic transmission executed by the prior art cruise control device is started only by the turning-on of a switch, such as the resume switch etc. Therefore, when the cruise control is carried out while the vehicle is ascending a long slope, for example, the shift operation of the automatic transmission cannot be promptly effected even if the vehicle speed V largely drops from the target speed VO. Namely, the automatic transmission carries out a shift change operation independent of the shift command signal from the cruise control device, only when the vehicle speed or the amount of depression of the accelerator pedal meets the shift change condition, as conventionally known. Moreover, drivers usually set their foot away from the accelerator pedal when the vehicle is cruising, and accordingly, the depressing of the accelerator pedal by the driver, i.e., the shift change operation of the automatic transmission, is inevitably delayed.
Thus, the prior art cruise control device is disadvantageous in that the shift change control of the automatic transmission cannot be effectively carried out in accordance with the running condition of the vehicle and that the cruise control is interrupted, thus failing to ensure a satisfactory driving feeling which is the inherent purpose of the cruise control.